Tolerant of knight that is breaking fast (7)
I believe the answer is:
lenient
'tolerant' is the definition.
(similar in meaning)
'knight that is breaking fast' is the wordplay.
'knight' becomes 'N' (chess abbreviation).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'breaking' means one lot of letters goes inside another.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'n'+'ie'='nie'
'nie' inserted into 'lent' is 'LENIENT'.
'of' acts as a link.
(Other definitions for lenient that I've seen before include "Forgiving, soft" , "Indulgent, lax" , "'Indulgent, easy (7)'" , "Forgiving, gentle" , "Tolerant, rather than strict" .)